In vehicles, particularly commercial vehicles, the foremost of a plurality of rear axles is driven, while the rear axles are purely trailing or lift axles and as such exhibit no drive function.
When starting a vehicle of this kind, the drive wheels may spin due to a lack of static friction, since the movement of the vehicle body during starting inevitably causes an additional load on the trailing axle and also a removal of the load from the drive axle. If this removal of the load from the drive axle falls below a given limit, it causes the drive wheels to spin.
The air pressure required in the bellows springs of the pressure chambers of the axles results from the load situation. The air pressure is distributed between the main and the additional axle based on the traction control principle, wherein each of the two rear axles is given its own permissible maximum pneumatic suspension pressure value, which corresponds to a fully laden vehicle. The aim of the traction control is primarily to load the drive axle as the load increases, namely by venting the pneumatic spring bellows of the drive axle up to the maximum permissible pressure, while maintaining the vehicle level. If there is a further increase in load, this is then borne by the additional axle, wherein the pressure in the bellows of the drive axle is held at the maximum permissible pressure.
The pressure in the bellows of the trailing axle is thereby reduced to a minimum value specified by law. A relay control ensures that the resulting high pressure in the bellows of the drive axle occurs only for a limited period. Once this time limit has been reached, this control switches the bellows of the trailing axle back to normal working pressure again. This means that an overloading of the bellows of the drive axle—primarily when the vehicle is fully laden—is avoided.
A process for the traction control of a vehicle with pneumatic suspension having a drive axle and a lift axle is disclosed in DE 10 2006 011 183 A1. A measurement of the pneumatic spring pressures on both vehicle sides of the drive axle and the lift axle is used to determine whether there is an overload on a vehicle side which means that the maximum permissible pneumatic spring pressures are exceeded.
In the event that the maximum permissible bellows pressures at the drive axle and the lift axle are exceeded on at least one vehicle side due to an overloading of the vehicle, the system switches into an overload mode, in which it is no longer the traction but a pressure ratio between the drive and trailing axle that is compensated according to the permissible bellows pressures at the two axles.
In order to retain at least largely the traction existing at the drive axle during the transition from traction control to pressure ratio control in overload mode, the overload is not sweepingly detected for the entire vehicle, but when an overload is identified on only one side of the vehicle, only this overload side is initially controlled according to the pressure ratio control in overload mode and the other lighter vehicle side is left in traction control with an adjustment. Due to the side-based overload detection, an overload is avoided by transferring the pressure from the drive axle to the trailing axle.
DE 35 45 222 A1 discloses a device for the partial removal of pressure from the trailing axle of a vehicle with a pneumatic suspension, which enables differentiated control of the pressure in the bellows of the trailing axle, which is adapted to the entire vehicle load. For this purpose, the pressure in the bellows of the trailing axle is controlled by the pressure in the bellows of the drive axle by means of a specific pressure ratio valve. In this way, synchronization either of the two axles as a fixed pressure ratio between the axles or the left and right vehicle side, respectively, of both axles relative to one another as a fixed pressure ratio for both axles on the left and right side of the vehicle is proposed.
A process for controlling a compressed air-based vehicle suspension having a plurality of pneumatic spring elements disposed between at least one vehicle body and at least one vehicle axle is disclosed by DE 38 15 612 A1. In this case, an excessive pressure differential between pneumatic spring elements is identified by means of a control device and a pressure compensation control process is initiated. It is thereby possible for an excessive pressure difference between pneumatic spring elements, for example at a vehicle axle between the pneumatic spring elements of the left and right side of the vehicle, to be eliminated without the level of the vehicle body and the load distribution between the individual groups of pneumatic spring elements being changed in an inadmissibly excessive manner. A pressure deviation in the different pneumatic spring elements should thereby be avoided.
In addition, DE 38 41 476 A1 relates to a level control in a vehicle suspension for a right and a left vehicle side. In order to prevent a vehicle side from remaining above the level when the level drops, because the weight of the body is almost completely borne by the first pneumatic spring element or is taken over by another axle, for example, a control device ensures that there is venting on the vehicle side on which the level already lies within a tolerance range, in order to adjust the mean vehicle level to a permissible value.